As a conventional apparatus for detecting an average luminance level of a video signal, there is known a video signal level detection apparatus that is disclosed in Japanese Published Patent Application No. Hei8-202319.
FIG. 8 is a block diagram illustrating the structure of the conventional video signal level detection apparatus.
This video signal level detection apparatus includes an input terminal 1001 for receiving luminance signal data that is separated from a video signal, an average calculation unit 1002 for calculating an average of pixel data corresponding to the inputted luminance signal data, for every n pieces, a data integration unit 1003 for integrating the data obtained by the average calculation unit 1002 for each screen, a division unit 1004 for dividing the data integrated by the integration unit 1003 by a constant that is obtained by dividing the number of pixels displayed on one screen by n, to detect an average luminance level, and a decoder 1005 for outputting the average luminance level.
The average calculation unit 1002 includes a first flip-flop circuit for latching the luminance signal data, an adder for adding pixel data of the luminance signal, a second flip-flop circuit for latching the added data, and a third flipflop circuit for extracting a predetermined number of bits starting from a Most Significant Bit (hereinafter abbreviated as MSB) of data that is outputted from the adder and outputting the extracted bits to the data integration unit 1003.
When the luminance signal data is inputted, the average calculation unit 1002 adds pixel data corresponding to the luminance signal for every n pieces, by means of the adder and the second flipflop circuit, extracts the predetermined number of bits starting from the MSB of the data that is outputted from the adder, and latches the data by means of the third flip-flop circuit, thereby calculating the average luminance level of pixel data corresponding to the luminance signal for every n pieces. The integration unit 1003 integrates the data obtained by the average calculation unit 1002 for each screen. The division unit 1004 divides the data integrated by the data integration unit 1003 by a constant that is obtained by dividing the number of pixels displayed on one screen by n, thereby detecting the average luminance level of the video signal.
According to the video signal level detection apparatus configured as described above, the average luminance level of pixel data corresponding to the luminance signal can be calculated for every n pieces. Then, on the basis of the calculated average luminance level, the average luminance level of the entire display screen is calculated. Therefore, the average luminance level of all pixel data can be detected with a small scale circuit.
The average luminance level of the video signal is often used as a modification effect control value when the picture quality is to be modified. In addition, with advances in the broadcasting technology, video signal formats have been increasingly diversified, and there are formats corresponding to 480i, 480p, 720p, 1080i and the like. Accordingly, it is required that the average luminance level of the video signal should be detected adaptively to the respective video signal formats, with great accuracy and without increasing the circuit scale.
However, when as in the conventional video signal level detection apparatus, pixel data corresponding to the luminance signal are added for every n pieces, the predetermined number of bits are extracted starting from the MSB of a signal indicating the addition result to calculate the average luminance level of pixel data corresponding to the luminance signal for every n pieces, and the average luminance level of the entire display screen is detected on the basis of this calculated average luminance level, the data concentrate more on lower-order bits in such a video signal format having fewer significant pixels and more higher-order bits remain unused, as the number of significant pixels in the video signal depends on the video signal formats, and therefore, the lower-order bits as effective values are cut down, thereby increasing a rounding error. Consequently, the accuracy of the average luminance level is decreased.
In addition to the apparatus disclosed in the above-mentioned application, there is commonly known a system in which, in order to reduce the circuit scale, luminance signals are thinned out to provide an average luminance level. However, in this system, pixels are taken out at the same positions on respective scanning lines, i.e., pixels are taken out from the scanning lines with a ratio of 1/8. Therefore, in cases where pictures of fixed patterns or repetitive patterns appear on the display screen, accurate average luminance levels cannot be detected.
The present invention is made to solve the above-mentioned problems, and has for its object to provide a luminance signal detection apparatus which can detect an average luminance level adaptively to various video signal formats, with a small scale circuit and with great accuracy.